Police and fire investigators say they are concerned about how a Hamilton home came to be destroyed by fire early yesterday, while witnesses believe a molotov cocktail was to blame.

A man who lives nearby said the flames at the Frankton address were like "a massive bonfire" and claimed an accelerant "must have been used".

"It wasn't just burning in one area.

"I mean, there were flames all over the place. It was massive, and it was hot," said the man, who did not want to be named.

Yesterday, investigators using accelerant detectors worked at the scene where the device could be heard sounding an alarm. Outside on the footpath a plastic sheet was placed over a patch of concrete where petrol was understood to have been found.

Northern fire communications shift manager Steve Smith said the house was well ablaze when firefighters arrived at the scene at 12.15am.

"We received multiple calls to the address where there were reports of flames coming through the roof," Mr Smith said.

Four appliances from around the city responded to the Norton Rd scene where neighbours were overwhelmed by the heat from the blaze.

The man who spoke to the Waikato Times said he rang 111 but was told others had already called.

"I was really worried about the woman that lives there. I thought she might be inside," he said. However, the house was empty at the time.

The blaze melted the spouting on a neighbouring property. Fire crew doused the next-door property to protect it from the flames and heat.

The man spoke to another neighbour who reported a "massive bang".

"So he came out to have a look and found this house up in smoke," he said.

The man said there was always "plenty of activity" at the address where police were frequently seen speaking with the occupants.

"Just last week the man that lives there was arrested and taken away.

"He put up quite a fight, too," he said.

Mr Smith said it took fire crew under an hour to bring the fire under control.